Implantable medical devices having electrical circuit components are well known in medical science. Some of the most common forms of such implantable devices are pacemakers and defibrillators. Additionally, implantable drug delivery systems are available for supplying needed medication for treatment of disease or for responding to the physiological demands of a patient in an emergency situation.
A pacemaker (or "pacer" as it is commonly labelled) is an implantable medical device which delivers electrical pulses to an electrode that is implanted adjacent the patient's heart in order to stimulate the heart so that it will beat at a desired rate. A normal human heart contains a natural pacemaker by which rhythmic electrical excitation is developed. If the body's pacemaker performs correctly, blood is oxygenated in the lungs and efficiently pumped by the heart to the body's oxygen-demanding tissues. However, when the body's natural pacemaker malfunctions, due to age or disease, an implantable pacemaker often is required to properly stimulate the heart. An in-depth explanation of certain cardiac physiology and pacemaker theory of operation is provided in U.S. Pat. No. 4,830,006.
In recent years, "rate-responsive" pacemakers have been developed which, in response to a sensed physiological parameter, will automatically change the rate at which the pacemaker provides stimulating pulses to the heart. The physiological parameter provides some indication of whether the heart rate should increase or decrease, as dictated by the physiological needs of the patient. For example, where the patient is at rest, the rate-responsive pacemaker will maintain a normal or base rate of, for example, 60-70 pulses per minute. However, if the sensed physiological parameter indicates that the patient is exercising, then there is a need for the heart to beat much faster, and the pacemaker will respond by stimulating the heart to beat at a higher rate, for example, 100-110 beats per minute.
Similarly, implantable defibrillators sense physiological parameters in order to determine when to supply a defibrillating shock to a patient's heart. Ventricular fibrillation is a condition characterized by rapid, chaotic electrical and mechanical activity of the heart's excitable myocardial tissue, and results in an instantaneous cessation of blood flow from the heart due to the uncoordinated or ineffectual action of the ventricles. Defibrillation is a technique employed to terminate fibrillation by applying one or more high energy electrical pulses to the heart in an effort to overwhelm the chaotic contractions of individual tissue sections and to restore the synchronized contraction of the total mass of tissue.
Likewise, implantable drug delivery systems also may rely upon physiological parameter sensors to provide signals that may be processed internally in order to determine when, and in what amount, a stored drug is to be delivered into the patient's body. In the treatment of certain diseases, it is desirable to administer a drug into a particular location within the body where the drug will be most effective in combating the localized disease. As another example, in treating cardiac arrhythmias, it is sometimes desirable to deliver the drug directly to the heart. In other applications of implantable drug delivery systems, the location at which the drug is introduced into the body is not critical, and the body's circulatory system is relied on to carry the administered drug to all parts of the body. Drugs that may effectively be administered by implantable delivery systems include insulin, glucose, heparin or any of a variety of chemotherapeutic agents.
Because the condition requiring the use of an implantable device may drastically impair the patient's quality of life or, in some instances, is a life threatening condition, having reliable indicators of physiological parameters is imperative. Physiological parameter sensors and activity parameter sensors that have been employed in association with implantable devices, or that those in the art have suggested may be employed, include those that sense respiration rate, blood oxygen saturation level, temperature, blood pressure, pH, length of the Q-T interval, the length of the P-R interval, thoracic impedance changes, nerve activity, biochemical concentrations (such as enzymes and glucose) and motion or acceleration.
Regardless of the sensed parameter, most prior art implantable devices have relied upon physiological parameter sensors that are positioned remotely from the implantable device. For example, U.S. Pat. No. 4,886,064 discloses sensors that are implanted remotely from a pacemaker and that wirelessly transmit to the pacemaker signals that indicate or correlate to a sensed parameter. In many other prior art implantable devices, however, the remote sensors are interconnected with the implantable device by means of electrical leads or conductors that are encased in a catheter that extends between the remote sensor and the implantable device. For example, U.S. Pat. No. 4,903,701 discloses an oxygen sensor that is located remotely from the pacemaker and that is mounted on the electrical leads used to transmit the generated pulse from the pacemaker to the stimulating electrode. U.S. Pat. No. 4,763,655 discloses a temperature sensor and a blood oxygen sensor that are implanted remotely from the pacemaker housing. The sensors are coupled with the pacemaker circuitry by conductors encased in a catheter.
Designs for implantable devices that rely upon remote sensors pose significant problems. First, radiofrequency transmission, as suggested by U.S. Pat. No. 4,886,064, typically requires transmitters and receivers that substantially increase the volume, weight and complexity of the pacemaker and sensor. These characteristics are generally undesirable in an implantable device where, for patient comfort, small size and light weight packages are desired goals. Second, remote sensors often need specialized catheters and are susceptible to fixation and migration problems. Another drawback of employing remotely-positioned sensors, at least those that are implanted within or adjacent to the patient's heart, is that should such a sensor fail, delicate surgical intervention may be required to remove and replace the faulty sensor. By contrast, pacemakers and many other implantable devices are typically positioned in an easily accessible location just beneath the patient's skin, and can be accessed and replaced without the risk of life-threatening or extremely costly surgery. Also of significance, because of the required electrical connections between the remote sensors and the internal circuitry within the implantable device, the device is susceptible to infiltration by corrosive body fluids. Any such infiltration will almost instantaneously disable the electrical circuitry in the device. Thus, the locations at which the sensor's leads penetrate the housing of the implantable device must be sealed to prevent infiltration of body fluids.
Those involved in the medical arts already have confronted the problem of preventing fluid from infiltrating into an implantable device at the locations where external leads attach to the device housing. As mentioned previously, it is conventional practice to surgically implant a stimulating electrode adjacent the heart and to interconnect the electrode to the pacemaker via conducting leads. This arrangement is shown, for example, in U.S. Pat. No. 4,903,701. Additionally, certain implantable medication delivery systems require that electrical conductors interconnect an implantable device containing a power source and control circuitry and a remotely positioned drug dispensing device as, for example, disclosed in U.S. Pat. No. 5,041,107. It is, of course, important that all such leads be securely attached to the implantable device to prevent the leads from becoming inadvertently decoupled. At the same time, because pacemakers and drug delivery systems require periodic replacement, and because this replacement procedure ideally is accomplished without disturbing any remotely implanted electrode or other device, the connections between the leads and the implantable device housing must be readily disconnectable. It is critical, of course, that the lead termination and attachment mechanisms prevent infiltration of any fluids into the implantable device.
To date, it has been common in the design of implantable devices to provide the device with a header portion which includes one or more terminals for landing and terminating any external leads. The header, which may be made of an epoxy material, supports and insulates the terminals. Internal conductors interconnect the terminal in the header portion with the electrical circuitry contained within the housing of the implantable device. Examples of such headers are shown in U.S. Pat. Nos. 5,282,841 and 4,072,154. Where physiological parameter sensors (rather than electrodes) are implanted remotely from the implantable device, similar such termination means must be provided for landing and terminating the conductors that communicate signals between these remote sensors and the implantable device.
In part due to the various problems and disadvantages of positioning sensors remotely from the pacemaker or drug delivery device with which they are associated, it has been suggested that the sensors be housed within the implantable treatment device itself. For example, U.S. Pat. No. 5,040,533 proposes an implantable cardiovascular treatment device having self-contained sensors and a window formed in the wall of the container to permit the sensors that are housed within the container to detect or measure a physiological parameter through the window. Although proposed as a solution to some of the aforementioned problems associated with external sensors, the "windowed" pacemaker presents additional and even more pronounced sealing problems. Because the interface between the window and the walls of the container must prevent infiltration by body fluids, a complete and enduring seal must be devised and installed along the entire perimeter of the window. Providing such a seal in the walls of the container presents significant manufacturing difficulties, especially considering the small size of the container that is typically employed in pacemakers. Further, the additional components and manufacturing time and effort that are needed to provide such a sealed window in the walls of the implantable device would increase substantially the cost of manufacturing such a device.
Thus, despite significant advances in the art that have been made over the years, there remains a need for an implantable device capable of housing and protecting any of a variety of types of physiological parameter sensors. The device must permit the sensors to carry out their intended functions and, simultaneously, must seal the devices from exposure to corrosive body fluids. Especially well received would be an implantable device that would be no larger than those presently employed and that would not require significant additional retooling in order to manufacture a new and specialized housing, such as one requiring that a window be formed in what would otherwise be a continuous wall. Ideally, the new device would not create additional sealing problems and could employ known and reliable electrical power supplies and circuitry. Preferably, the implantable device would also house a telemetry link through which data and instructions could be communicated to and from the implantable device.